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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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576 Chapter 19 Ordinary differential equations I

particularintegralinsuchacase,wecarryouttheprocedurerequiredwhentheauxiliary

equation has equal roots.Thatis, we tryy P

= αxe 3x . We find

y ′ =αe 3x (3x+1)

y ′′ =αe 3x (9x+6)

Substitution into the inhomogeneous equation yields

αe 3x (9x +6) − αe 3x (3x +1) −6αxe 3x =e 3x

Mosttermscancel, leaving

5αe 3x = e 3x

sothat α = 1 5 .Finally,therequiredparticularintegralisy P =xe3x 5 .Thegeneralsolution

istheny=Ae 3x +Be −2x + xe3x

5 .

Engineeringapplication19.9

Transmissionlines

A transmission line is an arrangement of electrical conductors for transporting

electromagnetic waves. Although this definition could be applied to most electrical

cables,itisusuallyrestrictedtocablesusedtotransporthigh-frequencyelectromagneticwaves.Thereareseveraldifferenttypesoftransmissionlines.Themostfamiliar

one is the coaxial cable which is used to carry the signal from a television aerial to

atelevisionset(seeFigure19.8).Whenahigh-frequencywaveisbeingcarriedbya

cableseveraleffectsbecomeimportantwhichcanusuallybeneglectedwhendealing

with a low-frequency wave. These are:

(1) the capacitance,C, between the two conductors,

(2) the series inductance,L, ofthe two conductors,

(3) theleakagecurrentthroughtheinsulationlayerthatseparatesthetwoconductors.

The electrical parameters of a coaxial cable are evenly distributed along its length.

Thisistruefortransmissionlinesingeneralandsoitisusualtospecifyperunitlength

values for the parameters. When constructing a mathematical model of a transmission

line it is easier to think in terms of lumped components spanning a distance δz

and then allow δz to tend to zero (see Figure 19.9). The leakage between the two

conductors is conventionally modelled by a conductance, G, as this simplifies the

mathematics and avoids confusion with the line resistance,R. Note thatC,G,Land

Rareper unit length values forthe transmissionline.

×

Outer conductor

Inner conductor

Protective coating

Insulator

Figure19.8

Acoaxial cable.

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